<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">

        #box1 {
            width: 100px;
            height: 100px;
            background-color: red;
            position: absolute;
        }

        #box2 {
            width: 100px;
            height: 100px;
            background-color: yellow;
            position: absolute;

            left: 200px;
            top: 200px;
        }

    </style>

    <script type="text/javascript">

        window.onload = function () {
            /*
             * 拖拽box1元素
             *  - 拖拽的流程
             * 		1.当鼠标在被拖拽元素上按下时，开始拖拽  onmousedown
             * 		2.当鼠标移动时被拖拽元素跟随鼠标移动 onmousemove
             * 		3.当鼠标松开时，被拖拽元素固定在当前位置	onmouseup
             */

            //获取box1
            var box1 = document.getElementById("box1");
            //为box1绑定一个鼠标按下事件
            //当鼠标在被拖拽元素上按下时，开始拖拽  onmousedown
            box1.onmousedown = function (event) {
                event = event || window.event;
                //div的偏移量 鼠标.clentX - 元素.offsetLeft
                //div的偏移量 鼠标.clentY - 元素.offsetTop
                var ol = event.clientX - box1.offsetLeft;
                var ot = event.clientY - box1.offsetTop;


                //为document绑定一个onmousemove事件
                document.onmousemove = function (event) {
                    event = event || window.event;
                    //当鼠标移动时被拖拽元素跟随鼠标移动 onmousemove
                    //获取鼠标的坐标
                    var left = event.clientX - ol;
                    var top = event.clientY - ot;

                    //修改box1的位置
                    box1.style.left = left + "px";
                    box1.style.top = top + "px";

                };

                //为document绑定一个鼠标松开事件
                document.onmouseup = function () {
                    //当鼠标松开时，被拖拽元素固定在当前位置	onmouseup
                    //取消document的onmousemove事件
                    document.onmousemove = null;
                    //取消document的onmouseup事件
                    document.onmouseup = null;
                };
            };


        };


    </script>
</head>
<body>
<div id="box1"></div>

<div id="box2"></div>
</body>
</html>
